Data communication networks for exchanging data transmissions between two or more data transmitting and receiving devices were popularized with the advent of electronic computers and the development of digital signal processing techniques. A typical data communication network arrangement interconnects a plurality of data transmitting and receiving devices to a data network via associated interfaces. The data communication network typically comprises a bidirectional bus which provides the medium for data transmission among the data transmitting and receiving devices. It is known to use packet-type data transmissions for the above-described communication network. The devices connected to the bus may comprise a variety of data transmitting and receiving equipment such as large scale computers, personal computers, intelligent and non-intelligent terminals or digital station sets. Each device connects to the network via an interface. The interface processes incoming and outgoing packet data transmissions to and from the devices. The processing operations include such processes as determining the origin and destination of the data packet, controlling access to and from the bus, and formatting and unformatting the data packets. Typically, a device, such as a terminal, is connected to a single interface which performs the processing operations associated with the receipt and transmission of data. However, an arrangement that utilizes a single interface for an associated data transmitting and receiving device is costly and inefficient. Therefore, arrangements were developed that utilize a single interface to perform processing operations for a plurality of associated data transmitting and receiving devices.
A shared interface arrangement is typically found in small business offices where economies are important. For example, a number of non-intelligent terminals located at each employee's desk are connected through a single interface to a bi-directional data packet bus. The bus is connected to a host computer via an associated interface. This arrangement provides all the employees having a terminal to access the host computer via the single interface. Therefore, all data transmissions to and from each terminal are processed by the single interface. The shared processing operations allow any of the terminal users to intercept any incoming or outgoing data transmissions merely by monitoring the current processing operations of the interface, and detecting the receipt or transmission of data.
Although a shared interface provides economies, an overwhelming disadvantage of the shared interface arrangement is that any data transmission, either received or transmitted, may be intercepted by any of the plurality of devices connected to the interface. Any device user may "eavesdrop" on a data transmission despite the fact that the user's device is not the originating device or destination device of the data transmission. The above-described arrangement, therefore, precludes the transmission and receipt of data having a proprietary nature to and from the network since each device is incapable of receiving or transmitting a secure data transmission. Heretofore, no arrangement exists which provides for secure data transmissions to and from a single data device where the single device shares a common interface to a network with a plurality of other data devices.